1. Field of the Invention
The invention relates to vinyl alcohol copolymers, water-soluble films containing them, which films can be used advantageously as packaging material or auxiliaries for packaging purposes, and to a method for producing the said water-soluble films, in particular such with controllable water solubility.
2. Description of Related Art
From EP-A2-283180 water-soluble films on the basis of polyvinyl alcohols are known, namely vinyl alcohol/vinyl acetate copolymers which are partly acetalated with benzaldehyde derivatives. These films are produced by the known film casting method, with the aqueous solution consisting of the acetalated polyvinyl alcohol being cast onto a moved carrier tape by means of a die. Thereafter the film is dried in the manner known per se and drawn off from the carrier tape. The water-soluble film produced in this manner is suitable for the packaging of various alkaline and acidic packed goods such as soaps, detergents or agricultural and industrial chemicals such as insecticides, herbicides, fertilizers, detergents or bleaching agents.
These films are provided with the disadvantage in connection with their use that they can release migration-capable toxic substances in contact with many packaged goods, in particular with alkaline or acidic reacting compounds from foodstuffs. Moreover, such films, which consist nearly completely of synthetic polymers, are provided with low biodegradability owing to their chemical constitution.
From EP-A-304 410 it is known to produce moulded bodies, in particular foils, from destructured starch. Although in this case it concerns a polymer consisting of natural materials, it can be degraded biologically, but only with a relatively low degradation speed. Accordingly, such films can only be disposed of biologically within long periods of time.
Furthermore, the U.S. Pat. No. 3,316,190 described mixtures which are soluble in cold water, consist substantially of polyvinyl alcohol and are obtained by mixing (a) polyvinyl alcohols or acetals produced therefrom, with (b) tensides and (c) optionally also water-soluble starch. In these mixtures the starch component does not contribute to the biological degradability, but only to the avoidance of residual adhesiveness of the films produced from this mixture.
From DE-A-2022875 mixtures of polyvinyl alcohol and starch are known which are used for the production of porous polyvinyl acetate objects.
From U.S. Pat. No. 3,737,398 formalin dialdehyde starch polyvinyl alcohols are known which are used for the surface treatment of metal objects.
Moreover, from U.S. Pat. No. 3,098,049 reaction products are known which are obtained by the conversion of hydrolized polyvinyl acetate polymers with periodate-oxidized polysaccharide. These reaction products are characterized by high resistance to water, i.e. they can be designed as insoluble in water.
Furthermore, it is known to use glass vessels with closures made from metal foils instead of foils as packaging material. The metal foils are attached to the glass vessels by means of lacquer sealing.
In reclaiming the glass vessels it should be possible to remove the sealing lacquer residues very easily. This is prevented in most cases however by the employed sealing lacquer as a result of its low water solubility.